The present invention relates to a reference voltage circuit, and more particularly relates to a reference voltage circuit suitable for stabilization of a reference output voltage against variations in power source as well as in current amplification factor (h.sub.FE) of transistors.
Heretofore, a band gap reference voltage circuit is known as a circuit for obtaining a stabilized reference voltage having a small temperature coefficient (reference is made to a book entitled "Shuseki Kailo Kogaku (2) (Integrated Circuit Engineering (2))", by Nagata & Yanai, published by Corona Co., pages 23 and 24). The circuit is shown in FIG. 1.
In FIG. 1, the reference voltage circuit comprises transistors 1 to 3, resistors 4 to 6, and a constant-current power source 7. The resistors 4 and 6 and the transistors 1 and 2 constitute a constant-current circuit which determines a current I.sub.2 flowing in the resistor 5. An output voltage V.sub.0 at an output terminal 100 is the sum of a potential difference V.sub.BE3 between the base and the emitter of the NPN transistor 3 and a terminal voltage across the resistor 5. Because the potential difference V.sub.BE3 has a negative temperature coefficient while the terminal voltage across the resistor 5 has a positive temperature coefficient, the resistor 5 can be suitably adjusted to make the whole temperature coefficient zero. If the current flowing in the resistor 4, the current flowing in the resistor 5, the resistance value of the resistor 5, and the resistance value of the resistor 6 are represented by I.sub.1, I.sub.2, R.sub.5, and R.sub.6, respectively, the output voltage V.sub.0 is expressed by the equation: ##EQU1## in which k represents Boltzmann constant, and q represents the quantity of electric charge of an electron.
In the conventional circuit, the NPN transistor 3 of FIG. 1 performs a function of stabilizing the output voltage. The transistor 3 operates to absorb variations of the circuit current caused by the factors, such as variations in power source (that is, variations of the output current of the constant-current power source 7), variations in load connected to the output, and the like, to thereby keep the currents I.sub.1, I.sub.2 and the like constant.
The output current I.sub.CC of the constant-current power source 7 is the sum of the current I.sub.1 flowing in the resistor 4, the current I.sub.2 flowing in the resistor 5, and the collector current I.sub.C3 of the NPN transistor 3 (the current I.sub.2 being the sum of the collector current I'.sub.2 of the NPN transistor 2 and the base current I.sub.B3 of the NPN transistor 3). In order to keep the output voltage V.sub.0 constant regardless of the change of output current I.sub.CC, current I.sub.1 should be constant or in other words current I'.sub.2 should be constant. Accordingly, the variations of the output current I.sub.CC should be reflected mainly in the collector current I.sub.C3 of the NPN transistor 3. The constant-current circuit composed of the resistors 4 and 6 and the transistor 1 and 2 has high impedance in the region of current larger than the collector current I'.sub.2 of the NPN transistor 2, so that when viewed from the constant-current power source 7, the resistor 5 and the base-emitter of the NPN transistor 3 are in the form of a series connection. Accordingly, the variations of the output current I.sub.CC cause variations of the base current I.sub.B3 of the NPN transistor 3, and hence, variations of the collector current I.sub.C3 which is the product of the base current I.sub.B3 and the current amplification factor h.sub.FE. In short, the variations in the constant-current power source 7 or the like are absorbed by the NPN transistor 3.
However, in fact, the current amplification factor h.sub.FE of the NPN transistor 3 has a finite value, and if the collector current of the NPN transistor 3 changes, the base current of the same changes corresponding to the value of the current amplification factor h.sub.FE. Because the current I.sub.2 flowing in the resistor 5 is equal to the sum of the collector current of the NPN transistor 2 and the base current of the NPN transitor 3, the current I.sub.2 changes if the base current of the NPN transistor 3 changes. If the current I2 changes, the temperature coefficient of the terminal voltage across the resistor 5 changes, so that the temperature coefficient of the output voltage V.sub.0 is not kept zero to thereby exert an influence on the output voltage.
In the following, an example of variations of the current I.sub.2 is described.
Assuming that the values of the resistor 5, the current amplification factor h.sub.FE of the NPN transistor 3, the base-emitter voltage V.sub.BE3 of the NPN transitor 3, and output voltage V.sub.0 are 6 k.OMEGA., 100, 0.7 V, and 1.2 V, respectivley, then the current I.sub.2 takes the value of about 833 .mu.A from the following equation. EQU I.sub.2 =(V.sub.0 -V.sub.BE3)/R.sub.5 ( 2)
If a current variation of 1 mA is applied under this condition, the variation .DELTA.I.sub.2 of the base current I.sub.2 of the NPN transistor 3 takes the value of 10 .mu.A from the following equation. EQU .DELTA.I.sub.2 =1 mA/h.sub.FE ( 3)
Accordingly, the variation in the terminal voltage across the resistor 5 takes the value: 6 k.OMEGA..times.10 .mu.A=60 mV, or in other words the terminal voltage across the resistor 5 changes by 5% with respect to output voltage V.sub.0 =1.2 V.
Although description has been made as to variations in output due to variations in power source as well as due to variations in load, it is to be understood that variations in output depend on the fact that the current amplification factor h.sub.FE of the NPN transistor used for stabilization of output is limited. Accordingly, variations in output voltage are not avoidable also in the case where the current amplification factor h.sub.FE changes.